|Copper Red Crystals|
Say it after Me
The two words, "long" and "weekend", when they fly in tight formation on the page or in conversation, usually have the blissful association of a three day holiday. Say it after me.... "L-o-n-g Weekend"..... and dream of a sunny day at the beach, or a restful few hours reading a book and dozing a little on a comfortable seat in the shade of a tree!
|"Dream Fish", by Laura.... There are other things going on in there as well!|
My Long Weekend
The weekend just past was a long weekend, but this was one with exactly the opposite meaning to the holiday one.
I suppose it started on Friday for me, with unloading a bisque firing from my electric kiln.
|Three bisque fired pots freshly unloaded, and Ginger the cat!|
Into the empty and still warm kiln I then loaded a glaze firing of crystalline glazed pots, and prepared the kiln to be able to do an oil drip reduction firing. In the kiln was a large vase, a small vase, a large bottle, and a small glaze test bowl, there was no room for anything else.... not even a cat!
To give you an idea of what a crystalline glaze firing is like, I have prepared a little summary of events from the times and temperatures that I recorded in my kiln log.
|"Mr & Mrs Moon - At Home", by Laura|
Friday November 4
I had the kiln underway by midday, but, because the glazes were still a bit damp, I simply took the kiln slowly to where the pots could gently steam out the moisture, and kept that going for the rest of the afternoon with the bungs removed. My kiln is essentially manual, but you can set a temperature for it to switch off at. So for this, I set it up to switch off at about 100C (212F). Then every hour or so through the afternoon, I would check progress and reset the kiln if it was off. That way I can maintain a reasonably constant temperature in the kiln. The kiln is well insulated so it is very slow to lose heat if it goes off when at that temperature.
6 in the evening
There was no steam coming from the top vent of the kiln, so I started the firing. I always test for steam by holding a mirror over the top vent of the kiln. If it fogs up rapidly, I know that there is still moisture in the pots to get rid of.
(I am a bit cautious in this, and I know that quite a lot of potters would do the early part of the firing faster, but I do not lose work with pots blowing up in the kiln, and I do not usually have problems with glazes crawling due to being put wet into a kiln and then fired rapidly.)
10.15 in the evening
The kiln is at 500 C (932F). I have medium power selected on each of the three simmerstats that regulate the power to the kiln elements, and can go to bed for a little while.
|"Ivy Tree", by Laura|
Saturday November 5
The kiln is now at 850C (1562F) and I move it to full power. I calculate the likely time for it to reach peak temperature by consulting previous firing logs, and set my alarm clock to wake me well before that, but I do not need it... Nigella Stopit wants to be fed just before 3am, and makes this known very forcefully!
The kiln is 1150C (2102F), and I calculate that we have about another 40 minutes to go. I reset the alarm.
I have been tossing and turning and not able to sleep. Something makes me get up just before the alarm. I find that the kiln has gone off and is showing 1250C (2282F) which is about 10 degrees below the 1260C (2300F) peak temperature that I was aiming for.
I am puzzled as the kiln must have come to peak temperature earlier than I thought. Actually, it is very easy to be puzzled at that hour of the morning, and I make myself understand what I am observing and force myself to think about it. I remove the bungs from the spy holes in the kiln and check the cones that I have set. Cone 8 is down, but cone 9 is still resolutely standing. Cone 9 should be down. I re-check to make sure that I am seeing things correctly, and it really is cone 9 that is standing tall. I switch the kiln on again, and keep a good watch until 9 does go down.
(What has probably happened is that we may have suffered a minor power cut. Even a brief interruption to our power supply will trip the kiln off, and it does not reset automatically. I am thankful that I almost always use cones when I do a glaze firing. Under-firing this by 10 degrees would have ruined the firing as the top temperature is very critical for crystalline glazes. I am very, very thankful for cones!).
1260C (2300F) Cone 9 down and kiln off.
The kiln is at 1100C (2012F) and I restart it and put it on a medium setting that I know will do a reasonable job of holding the temperature steady.
The kiln is holding 1100C (2012F) nicely, so I know that I can go to bed for a half hour rest before checking again... and again every half hour or so!
I switch the kiln off, and allow the temperature to fall to 1045C (1913F). I am doing this dip in temperature to make a growth ring form in the crystals that should be growing on the pots. Sometimes I will do as many as 4 or more of these dips throughout the time that the crystals are growing, but for these pots I just want simple crystals with a growth ring near the outer margin.
I turn the kiln on to a low setting and hold 1045C (1913F) for about 5 minutes then put the kiln on full power
Back at 1100C (2012F), readjust the kiln to hold this temperature. By this stage of the firing I generally find that I have to use slightly more power to maintain the holding temperature.
I repeat the dip in temperature and the climb, but this time switch the kiln off when nearly back to the holding temperature. I am hoping that this little dip and climb will do something interesting to the margin of the crystals that I have been trying to grow in the glazes.
|The temperature changes helped produce the decorative margins to the crystals.|
Kiln at 1065C (1949F) I switch off.
The kiln is at 820C (1508F), and I turn on a line that takes cooking oil into the kiln. I calibrate it carefully so that it is dripping cooking oil at a rate of about one drip per second. I am doing this to create an atmosphere in the kiln that is depleted of oxygen and high in carbon monoxide. This should have the effect of stripping oxygen atoms from the metals in the glaze, and making a dramatic change to the colour of the glaze. For example, a pretty copper green glaze can turn red, or even to shiny copper metal.
|These crystals are red, but also have a metallic shine.|
The kiln is at 615C (1139F) and I turn the oil drip off. Not much evidence of smoke through all the reduction period of this firing, and I am anxious that I did not have quite enough oil going through. I wanted to stick to the 1 drip per second though, as..., if I fiddled as I usually do, I have nothing to tell me in future what amount of oil was needed.
|"Christmas Tree", by Laura|
Sunday November 6
1pm the kiln is just below 200C (392F), and I risk a quick peep at the pots through the kiln lid (my kiln is top loading).
Hummm, sadly I was right, there really hadn't been enough oil dripping into the kiln, and what there was has only had a very minimal effect on the glazes. I consult my firing log book and work out that there are still enough hours left in the day to take the kiln back to 800C and do the oil drip reduction again. I shut the lid and turn the kiln back on!
The rest of the afternoon I am checking progress every half an hour and making necessary adjustments of the simmerstats to achieve a brisk, but not crazy, rate of climb.
The kiln is at 815C (1500F), I switch it off and I begin oil drip reduction again. This time more oil... quite a lot more!
Checking about once every 20 minutes, I keep the kiln in a smoky reduction atmosphere until it is down to 645C (1193F). Sometimes when I check the kiln, the kiln shed has smoke creeping out from gaps in the roofing iron.
Oil drip off. I can shut the kiln shed door and ... maybe get some sleep.
Monday November 7
1pm kiln just below 200C (392F), I risk a peep through the lid. This time the results look spectacular. I close the lid and write out the details of one of the pots on an exhibition entry form. Closing date for written entries 4pm Monday November 7. I make myself look almost presentable, and drive to town with my entry form, handing it in one and a half hours before the closing time.
|Vase, 15 inches (381mm) high|
After that I buy clay from Southern Clays in Dunedin. I deliver plant pots that I have made to a friend who commissioned them. Then I attend a 4.30pm monthly meeting of the Potter's Co-operative. Home by 8pm after shopping for groceries.
|Vase, 6.5 inches (165mm) high. (My favorite from the firing)|
Somewhere over the Friday and Saturday I installed and set up a Debian Linux operating system for my father on his computer, and it was lovely to see him on Saturday when he came to pick it up from me. Setting up a computer made a nice parallel activity to firing the kiln and thinking about pots and glazes. I do "unwind" sometimes by installing operating systems! Happy to report that his computer appears to be going well!
|The three pots from the kiln|
As I write this on Wednesday evening I am starting another crystalline glaze firing of the kiln!
*the close up photographs with this post are details of crystals that are on the little vase and the large vase that went through this firing. It is interesting how the size of a crystalline glazed pot does affect the shape and size of the crystals that grow on it. Part of the explanation for this is that the small pot may gain and lose heat more rapidly than the large one. Another explanation might be due to the placement of the pot in the kiln and what the temperature is in the space that it occupies.
**bisque fired pots. These are pots that have had a firing in the kiln up to a temperature that is high enough to make them strong enough to glaze easily. I bisque fire my pots to 1000 C (1832F), and currently glaze fire my crystalline glazed pots to about 1260C (2300F)
***crystalline glazed pots. These pots usually have a lot of zinc oxide in the glaze, between 20 to 30 percent. This much zinc will just about all go into solution in the glaze when it is at the peak temperature, but as the kiln begins to cool, the zinc is desperate to form crystals. By carefully controlling the peak temperature of the firing, the potter can determine the quantity of crystals that will form, by controlling the cooling of the kiln, the crystals can be grown to a particular size, and the shape can be influenced too. Crystalline glazing is an art and a science, and it requires enormous dedication and lots of testing. It is not for the faint hearted!